There have been practically used force detectors in which a force is applied to a spherical force transmission member and transmitted to a force sensor element, which in turn outputs a signal according to the magnitude of the transmitted force. The spherical force transmission member and the force sensor element are received in a casing. An uppermost portion of the spherical force transmission member protrudes outwardly out of a through hole formed in a top wall portion of the casing. The force sensor element is disposed between an inner surface of a bottom wall portion of the casing and a lowermost portion of the spherical force transmission member. At the bottom wall portion (or a side wall portion in the vicinity of the bottom portion) of the casing, input-output terminals are connected to the force sensor element and externally exposed. A substrate is disposed on an outer surface side of the bottom wall portion. The input-output terminals are soldered to the substrate. Thus, the force detector is mounted on the substrate.
As disclosed in JP2011-169717A (Patent Document 1), a soldering portion is provided at an opposite position to the lowermost portion of the spherical force transmission member in order to eliminate or reduce unevenness in detection accuracy due to bending or deflection of the bottom wall portion of the casing when soldering a small-sized force detector onto the substrate. At the soldering portion, the bottom portion of the casing is soldered onto the substrate, thereby filling a gap between the bottom wall portion and the substrate as caused when soldering the input-output terminals of the force detector to electrodes disposed on the substrate. This prevents a part of the applied force from being absorbed as the bottom wall portion located below the force sensor element is deflected when a downward force is applied to the force sensor element via the spherical force transmission member located above the force sensor element. Thus, unevenness in detection accuracy can be suppressed.
The principle of how to prevent unevenness in detection accuracy is illustrated in FIG. 3. FIG. 3 illustrates only the bottom wall portion 42 of the casing. As illustrated in FIG. 3a, the force sensor element 40 is disposed at a central portion of the inner surface of the bottom wall portion 42 of the casing. As illustrated in FIG. 3b, soldering land patterns are formed on an outer surface of the bottom wall portion 42. According to the soldering land pattern formed on the substrate, the force sensor element is connected to soldering electrodes, not illustrated, via solder portions 44, 46 on the substrate 48. FIG. 3b illustrates the land pattern formed on the outer surface of the bottom wall portion 42. Four land patterns 50 are disposed respectively in the vicinity of four corners of the outer surface of the square bottom wall portion 42 and work as input-output terminals of the force sensor element 40. The four land patterns 50 are connected respectively to the electrodes disposed on the substrate 48 by solder portions 44. In contrast, the land pattern 52 is not electrically connected to the force sensor element 40 and works to fill a part of the gap between the outer surface of the bottom wall portion 42 and the substrate 48 with a solder portion 46. Namely, since the bottom wall portion 42 is supported with respect to the substrate 48 only by the solder portions 44 in the vicinity of the four corners, a central portion of the bottom wall portion 42 would be deflected toward the substrate 48 without the solder portion 46 located at the position of the land pattern 52 when a downward force f is applied to the force sensor element 40 as illustrated in FIG. 3a. Thus, at least a part of the downward force f would be absorbed, thereby causing unevenness in detection accuracy. With the solder portion 46, the bottom wall portion 42 will not be deflected, thereby enabling the force sensor element 40 to output a signal accurately according to the magnitude of the force f.
JP2011-220865A (Patent Document 2) discloses a configuration in which fixing terminals are provided on the back surface of a package substrate of a force sensor package, planarly overlapping adhesion fixing positions of a sensor structure. Especially in FIGS. 6b and 6c illustrating a specific configuration, an SMD terminal 31 and a fixing terminal 32 are integrally provided to constitute a common terminal 33.